Optimization of rainwater harvesting system design for smallholder irrigation farmers in Kenya: a review

Climate change has significantly affected the livelihood and income of farmers across the globe (IPCC 2014). In sub-Sahara Africa (SSA), climate variability and change are considered a great threat to agriculture. Small-scale farmers who are vital in ensuring food security in the region are particularly more vulnerable to the adverse effects of climate change such as drought and flood (Verchot et al. 2007). During drought for instance, water sources for irrigation dry up leading to crop failure and subsequent famine resulting in social, economic and environmental losses (Mulwa et al. 2016). Under some extreme circumstances, the governments are prompted to provide food aid to cushion the population against malnutrition (Mburu et al. 2015). Climate change effects have further caused strain to the limiting water resources in SSA hindering the ability to expand irrigation (Vincenza et al. 2016).

To develop resilience, farmers have adopted different techniques such as rainwater harvesting (RWH), irrigation and planting of drought-resistant crops (Mburu et al. 2015). RWH in this context can be described as ‘a method of inducing, collecting, storing & conserving local surface runoff for agriculture production’ (Ibraimo 2007). RWH has the potential of increasing rain-fed agriculture production as well as reducing effects of soil erosion. It also allows crop production beyond the growing season through supplemental irrigation (Li et al. 2000). RWH can also be an effective source of water for domestic water supply in areas with a limited water supply such as arid and semi-arid areas. It can be easily collected and used without significant treatment (Nolan & Lartigue 2017). A study by Qin et al. (2019) proved that rainwater buffer tank significantly reduced the runoff peak flow hence had the capacity to protect against the adverse effects of flood such as damage to properties and loss of life.

Appropriate RWH techniques coupled with an efficient Decision Support System have become indispensable tools for conducting sustainable agriculture in the face of climate change (Fenu & Malloci 2020). The significance of RWH in this context is to make use of the available rainwater that is experienced in Kenya to increase crop production and enhance resilience to climate change. Research has shown that RWH can be effective in increasing crop production and improving adaptability to climate shock. For instance, a study by Kahinda et al. (2007) showed that supplemental irrigation of maize in Zimbabwe using rainwater increased yields and reduced the risk of crop failure. The collection and use of rainwater is therefore seen as a suitable remedy for adverse effects of drought among small-scale farmers.

Despite these known benefits, the adoption of RWH systems by small-scale farmers in Kenya for irrigation is relatively slow (Annastacia 2018). A study by Matiti (2018) showed that many RWH systems in Kenya are not performing as expected. The study indicated that a number of RWH systems are not reliable and are technically inefficient. Furthermore, a report by Chamwada (2019) indicated that a number of farm ponds and reservoirs constructed under the government household irrigation and water storage program do not fill to their capacity even after prolonged rainy seasons.

Wachira (2013) also noted that many smallholder farm ponds suffer high water losses through seepage and evaporation such that the ponds dry up before the end of the growing season. A study by Matiti (2018) found out that a good number of reservoirs have reduced capacity as a result of heavy siltation while some are completely damaged by water that overtops their banks leading to collapse. Consequently, the impact of the systems has been minimal with farmers taking relatively a long time to return their investment, while other investments on the same are completely lost as a result of the system failure (Kiggundu et al. 2018).

This review aimed at exploring the factors behind the poor performance of the many RWH systems in Kenya while attempting to render solutions for better design and implementation of the system in the region. It provides guidelines for the effective design and construction of an optimal RWH system for smallholder irrigation farmers. This will help designers and implementers of RWH systems to understand various approaches that can be applied to optimize the design parameters of the systems in order to contribute to better performance and improved adoption by smallholder farmers in the region hence enable them to develop resilience to climate shock.

The review is structured as follows: First, a brief description of RWH techniques used by smallholder irrigation farmers in the region is provided. Secondly, factors affecting the performance of RWH systems used by smallholder farmers have been explored. Finally, the review looked into the design aspects for an optimal RWH system for the region; consideration was made on ways of optimizing the design parameters to improve the RWH systems reliability, technical feasibility as well as the water-saving efficiency in the area. At the end, conclusions and recommendations are provided.

There are basically two methods of harvesting rainwater commonly used by smallholder farmers in Kenya, namely in situ and ex situ RWH. In situ RWH techniques involve capturing and using the rainwater at the farm through employing methods that increase the amount of water stored in the soil. Commonly applied methods include contour bunds, use of pits and strip catchment. Other methods of conservation agriculture such as deep tillage and contour farming are also classified under this technique (Hatibu & Mahoo 1999).

Ex situ RWH involves capturing rainwater outside the farm. The techniques involve the collection and storage of rainwater in natural or man-made reservoirs such as ponds, wells, small earth dams and other cisterns of different sizes for supplemental irrigation (Moges et al. 2011). They may be accompanied by intermediate storage for later use in supplemental irrigation (Black et al. 2012). This review will focus on ex situ RWH techniques, especially the construction and performance of farm ponds and reservoirs for small-scale irrigation.

A number of ex situ RWH systems have been used for smallholder irrigation in Kenya. Most commonly applied methods include water pans, rock catchments, earth dams and water ponds (Aroka 2010). Smallholder farmers across Kenya have dug water pans and ponds to capture and store runoff for irrigation. The pans and ponds are basically small earth dams of shallow depths ranging between 1 and 3 m with a dug stream and an embankment (Kimani et al. 2015). Several interventions have been carried out by the Kenyan government as well as other development agencies to promote RWH pans and ponds across the country (Chamwada 2019). Notable programs include a household irrigation and water storage program that is implemented by the National Irrigation Authority, Billion dollar alliance project; a partnership project between the Kenyan government, World Bank and World Agroforestry (ICRAF) for RWH in various regions of the country (Onyango 2017).

The National Irrigation Authority through the household irrigation and water storage program has implemented over 2,363 water pans in 12 counties since its inception (NIB 2019). Individual farmers have also been active in promoting RWH techniques in Kenya; this is evidenced by a number of farmers taking up the construction of ponds for irrigation across the country as well as the rise in manufacturing and importation of the geomembrane for pond lining in the country (Chamwada 2019).

Rock catchments that involve directing runoff from rock catchment surface to a reservoir usually consisting of a masonry wall have also been used in some semi-arid parts of Kenya. In some areas of Kitui and Makueni counties of Kenya, rock catchment and storage have been implemented to boost crop production through irrigation as well as to provide water for domestic use (Kimani et al. 2015). The stored water can be conveyed by gravity or pumping for irrigation to adjacent farmlands (Matiti 2018).

Smallholder farmers have also used hand-dug wells to store rainwater for supplemental irrigation. The wells have been constructed mainly in areas, where the water table is relatively high and at the river beds of seasonal rivers (Matiti 2018). They have been effective in storing rainwater during wet seasons for supplemental irrigation in dry periods. Water from the well can be manually lifted or pumped to a raised reservoir for subsequent application to irrigation fields (Aroka 2010).

Ex situ RWH techniques have been in existence in Kenya for many decades. Farmers and households have used the techniques to supplement rain-fed agriculture as well as augment the available water resources (Black et al. 2012). In the last three decades, there has been increasing interest toward the promotion of ex situ RWH systems in Kenya. The national governments, NGOs and other development agencies have initiated a number of projects to harness and store rainwater for irrigation in Kenya. However, many of the RWH systems constructed are not performing as expected (Matiti 2018). As a result, their adoption by smallholder farmers in Kenya is relatively slow. This can be attributed largely to technical and socioeconomic factors. Table 1 summarizes the key factors affecting RWH systems adoption by smallholder farmers in Kenya.

Technical inefficiencies in design and construction have been noted to contribute significantly to the inability of RWH systems to meet the expected water demand (Annastacia 2018). In addition, many RWH system implementers including individual farmers and organizations, lack access to adequate construction guidelines to enable them to make proper decisions with regard to the implementation of the RWH systems (KRA 2010). A study by Munyao (2014) revealed that a number of smallholder farmers lack technical skills for proper siting and construction of an appropriate RWH system. Consequently, a number of smallholder farmers in Kenya have ended up constructing RWH systems without any regard to proper design procedures making the systems prone to failures.

According to a study by Wanyonyi (1998), inadequate design hinders farmer's ability to maximize the benefits of RWH systems. Poorly designed farm ponds do not fill to their capacity even after prolonged rainfall. Improper siting of farm ponds leads to inadequate catchment resulting in low runoff collection. As result, a number of structures constructed for RWH do not sustain the crop water demands for the area (Matiti 2018). Appropriate RWH system design is therefore necessary to ensure that there is enough storage to meet the expected demand during the period of scarcity (Munyao 2014).

A suitable selection of design rainfall is required to properly size the storage in relation to the catchment area for effective RWH (Mati et al. 2005). If the rainfall in the area is less than the design rainfall, the stored water cannot meet the crop water requirement. However, if the design rainfall exceeds the expected amount, the system could be damaged and at times crops may be lost as a result of flood (FAO 2014a, 2014b).

Seepage and evaporation water losses have likewise significantly affected the performance of RWH systems in Kenya (Wachira 2013). The losses may account for 20–30% of the total volume (FAO 2010) and are more aggravated in arid and semi-arid areas that are characterized by loose soils and extreme weather conditions (Kimani et al. 2015). According to a study by Gachene & Kimaru (2003), a greater portion of arid and semi-arid lands of Kenya, where RWH is practiced, consist of black cotton soils (vertisols) which have a cracking tendency especially when dry hence cannot hold water during the wet season. A report by Chamwada (2019) further indicated that the majority of farm ponds implemented under the government water harvesting program have failed to supply the expected water demand owing to high seepage and evaporation losses. Table 2 shows seepage losses for various soils.

In addition, RWH systems for irrigation have also been limited in a capacity as a result of heavy siltation. A number of farm ponds and small earth dams constructed across the country have been abandoned due to reduced capacity caused by the deposition of sediments (Thome 2005). A study by Karara (2018) on the Kabiruini dam in Nyeri, Kenya noted that heavy siltation significantly hampered the dam's ability to supply the demand water for irrigation. It indicated that the dam depleted within two years of its desiltation due to sedimentation from the overlying catchment hence affecting substantially its productivity. Matiti (2018) further noted other challenges affecting the performance of RWH systems in Kenya including lack of requisite facilities such as animal watering troughs, poor accessibility, lack of proper intake and water draw-off structures, and unavailability of protective fence around many water points. The study proved that many water systems therefore suffered damage and contamination by animals as well as people during access limiting their usefulness.

Lack of efficient management and effective utilization of the limited stored water is also an impediment to the success of many RWH systems in Kenya. The absence of proper management strategy has contributed to the failures of many RWH systems in Kenya (Ngigi et al. 2005). Many smallholder farmers lack training in pond management such as essential skills in controlling rapid siltation and protection of pond liners (Munyao 2014). A study by Black et al. (2012) noted that a number of the farmers with installed pond liners suffered damage to the liner during the installations and operations compromising the usefulness of the liner.

Finally, lack of economic diversification in the utilization of the impounded rainwater has been identified as a major setback to the success of RWH for irrigation in Kenya (Matiti 2018). Chamwada (2019) also noted that a number of RWH systems have been constructed without regard to farmer's knowledge of irrigation methods as well as crop production. As a result, many farmers have used the stored water carelessly planting crops of low market values using water inefficient methods.

The main objective of RWH system for irrigation is to ensure the availability of an adequate amount of water for crop production. Effective design of RWH system therefore encompasses the areal precipitation, catchment surface area, water storage as well as the intended water use for the system (Rozaki et al. 2017). Studies have shown that rainfall intensities and distribution in a number of catchments in Kenya are adequate to produce sufficient runoff to sustain crop production during the dry season (Karara 2018; Matiti 2018). However, Matiti (2018) highlighted the poor performance of RWH systems making them unable to meet the crop water requirement in many cases.

Designing a suitable RWH system thus calls for the optimization of design parameters including the climatic, geological, hydrological and hydraulics conditions of the specific area under consideration for the maximum benefit (FAO 2014a, 2014b). An optimal RWH system is considered to be reliable, technically feasible and possesses a high water-saving efficiency (Matiti 2018). An appropriate design to achieve optimum standards therefore requires a systematic determination of climatic, hydrological and hydraulic parameters as well as an understanding of ways of optimizing the design parameters for maximum benefits (Mekdaschi Studer & Liniger 2013).

For optimal design in tropical humid regions such as Kenya, the use of long-term series of rainfall is recommended with a rainfall magnitude that occurs with 90% frequency. If the rainfall is less than the design rainfall, the stored water cannot meet the crop water requirement. If the design rainfall exceeds the expected amount, the system could be damaged and at times crops may be lost as a result of flood (FAO 2014a, 2014b).

where z = 254 for metric units, and RCN is the runoff curve number (estimated from tables).

For optimal design and operations, there is need for careful selection of the design rainfall for the catchment. The RWH system design should take into consideration the maximum potential rainfall (P) that is experienced at 90% frequency. The design must also take into account careful consideration of the appropriate runoff curve numbers that should be an accurate representation of the land-cover type and hydrologic condition of the catchment area. The maximum volume of the pond (V) can therefore be determined that can hold the desirable quantity of runoff (Q) from the catchment area and satisfies the crop water requirement (D_t).

RWH systems design can also be optimized for technical feasibility. Under this consideration, the design should try to answer the question ‘can it be done with the prevailing resources?’ (Senkondo et al. 2004). For RWH systems, technical feasibility is largely a factor of the precipitation that is experienced within a given catchment (Matiti 2018). If the rainfall experienced within an area is considered adequate, there exists a potential for harvesting and storage. The adequacy of rainfall for harvesting and storage is determined by establishing the design rainfall of a given catchment area (Mati et al. 2005).

The RWH system is feasible when the NPV is positive (Stec & Martina 2019).

IRR is the discount rate that makes the NPV of all cash inflows of a project equal to zero. A feasible project is one whose IRR is positive and higher than the decided discount rate. If the IRR calculated is greater than the reference rate, then the adoption of RWH system in the area is economically viable. If the IRR determined is less than the chosen reference rate, then the construction of RWH system in the area is economically unattractive (Rozaki et al. 2017).

B/C is also used to analyze the economic viability of RWH system investment. The costs and benefits associated with the project are spread throughout the life cycle of the project. For RWH systems, initial costs involve the costs of constructing the system, while operations and maintenance costs are incurred during the project's active life (Matiti 2018).

Benefits of RWH systems can be estimated both onsite and offsite. Onsite benefits in this context involve the quantity of crops harvested and their associated value, while offsite benefits may be approximated based on the opportunity cost such as flood protection and soil conservation (Senkondo et al. 2004). B/C is the ratio of discounted benefits to costs. If the value is more than 1, then the project is acceptable from the financial perspective; while a value less than 1 shows that the project has a negative financial return hence not acceptable (Rozaki et al. 2017).

where B_t is the benefit at time t, r is the discount rate and C_t is the initial cost at time t.

The design of RWH systems therefore calls for the optimization of design parameters to achieve maximum economic benefits.

Maximizing on water-saving efficiency of RWH system can be achieved through a number of techniques including optimal siting of the RWH system, appropriate design of the pond capacity, seepage and evaporation losses control as well as effective management of the stored water (Matiti 2018). The RHW system location should allow for the maximum collection and storage from the catchment. The size and dimensions should be designed such that the RWH systems can collect and store a significant amount of water for crop production (Mati et al. 2005). An optimal RWH system design allows for the maximum collection and storage to meet the demands of the crops throughout the rainfall distribution period. The storage capacity should be adequate to optimize the planting days after the end of the effective rainfall (FAO 2014a, 2014b).

Storage capacity can also be maximized by minimizing losses through seepage and evaporation (Rozaki et al. 2017). To control pond seepage, appropriate lining material can be used depending on costs and availability. Lining using soil base consisting of clay blanket is often preferred in many parts of Kenya because it is cheap and locally available (Fox et al. 2005). For small RWH systems, synthetic liners such as geomembrane may be used to prevent water leaks. In some cases with high evaporation, shade covers may be applied (Mati et al. 2005).

A number of RWH systems in Kenya have been limited in capacity due to siltation (Karara 2018). For optimal design and operations, RWH systems should have appropriate silt control structures to trap sediments and ensure the maximum utilization of the storage capacity (Matiti 2018). Finally, to maximize the benefits of RWH systems, it is prudent to incorporate an appropriate water use technology. A study by Njuguna (2014) found that optimum benefits of RWH systems for irrigation are achieved by integrating the system with water-efficient technologies such as drip irrigation.

The potential benefits of RWH in Kenya cannot be over-emphasized. It can provide the needed additional water resource to bridge the gap in water scarcity; support rain-fed agriculture through supplemental irrigation as well as reduce the adverse effects of climate change such as drought and flood. The Kenyan government and other development partners have been active in promoting these RWH technologies in the recent past. Furthermore, individual smallholder farmers have also shown a keen interest in adopting the RWH system techniques as a solution to irrigation water challenges in the last decade. This has seen a rise in the construction of water pans, small earth dams, rock catchments, shallow wells and ponds in different parts of the country.

However, these interventions have achieved little impact owing to consistent poor performance in many areas. From this review, a number of factors were noted to contribute to this low performance and include inadequate design of RWH systems, seepage and evaporation losses, siltation, lack of effective operation and maintenance program, ineffective management as well as inefficient utilization of the stored water. Proper design and implementation are therefore essential for better performance and adoption of RWH systems in the region. This will ensure that RWH systems are reliable, technically and economically feasible as well as possess a desirable water-saving efficiency.

Research on ways of improving the RWH system performance to maximize benefits is thus necessary. RWH system designers should therefore explore techniques of optimizing the design parameters including geological, hydrological and hydraulic parameters for each catchment under consideration to achieve the desirable RWH systems reliability, technical feasibility and water-saving efficiency.

All relevant data are included in the paper or its Supplementary Information.